Download Analytical Chem 7 SOLUTIONS

Analytical Chemistry 7 - Problems
A7 and A10: Chromatography
1.
Gas-liquid chromatography (GLC) is a powerful analytical technique. Outline the
principles of this technique, from the injection of the sample.
- sample is heated to vaporize it  but sample must not decompose at high temperatures
- mobile phase = inert gas like nitrogen, helium or argon
- stationary phase liquid long chain alkane adsorbed on solid support;
- different sample components carried through apparatus in stream of carrier gas
- each component has a different attraction for the hydrocarbon, depending on polarity and
molecular mass
- components move through at different rates - have different retention times
- more volatile components move through faster and have shorter retention times
- components converted to ions by flame ionizer as they leave the column
- charged ions can be converted to electrical signal by the detector
2.
Paper chromatography may be used to separate a mixture of sugars.
(a)
State the stationary phase and an example of a mobile phase used in paper
chromatography.
Stationary phase:
water in the paper
Mobile phase:
water or other solvent or solvent mixture
(2)
(b)
The identity of two sugars in a mixture can be determined by measuring their
Rf values, after staining.
(i)
Describe how an Rf value can be calculated.
(1)
Rf =
distance travelled by substance
distance travelled by solvent front
(ii)
(c)
Calculate the Rf value for sugar 2 in the chromatogram
at the right.
Explain how the Rf value of sugar 2 could be used to identify it.
- compare Rf of unknown to Rf of known samples
- when paper chromatography is carried out under the same conditions
(same mobile and stationary phases, and same temperature), different
substances have unique Rf values
3.
A mixture of two alcohols was analysed using high-performance liquid
chromatography, HPLC, and produced chromatogram 1 below.
a) In the space provided in chromatogram 2, sketch the chromatogram if the
column of the chromatograph was less tightly packed and all other variables were
kept constant.
Chromatogram 2 should be:
two broader peaks as chromatogram 1 for each peak;
two peaks of same area as chromatogram 1 for each peak;
two peaks of the same area as chromatogram 1 are closer to each other;
AND eluted earlier
b) Suggest, with a reason, a difference between alcohol 1 and alcohol 2 that would
account for the different retention times.
Alcohol 1 is less soluble in the stationary phase (or more soluble in the mobile
phase) than alcohol 2 so it moves through the column faster.
4.
(a)
Identify a possible mobile phase and stationary phase for HPLC and gasliquid chromatography, GLC.
Chromatographic
technique
Stationary phase
Mobile phase
HPLC
silica (silicon dioxide)
with adsorbed long
chain hydrocarbon
hexane or water or
alcohol or other liquid
solvent
GLC
long chain alkane on
column
inert N2, He or Ar gas
(4)
(b)
Deduce which technique, HPLC or GLC, can be used to analyse the urine
sample of an athlete for the anabolic steroid, tetrahydrogestrinone, THG.
(1)
GLC - commonly used technique in drug testing, and steroids do not
decompose at high temperatures
SEE the clarification notes in the Assessment Statements:
10.2 Deduce which chromatographic technique is most appropriate for separating
the components in a particular mixture.
Aim 8: HPLC can identify compounds that are temperature-sensitive. Uses include:
analysis of oil; alcoholic beverages; antioxidants, sugars and vitamins in foods;
pharmaceuticals; polymers; biochemical and biotechnology research; and quality control
of insecticides and herbicides.
GLC can identify compounds that can vaporize without decomposing. Uses include:
analysis of urine samples from athletes for drugs, underground mine gases and blood
alcohol levels.
NOW YOU KNOW!
(c)
Outline how the technique selected in part (b) would be carried out to confirm
the presence of the steroid THG in the urine sample.
- measure retention times for THG and sample
- if THG present in sample then retention times will be the same under the same
conditions
(2)
5.
An analgesic tablet contains 400 mg of aspirin and 80 mg of caffeine. The molecular
formula of aspirin is C9H8O4 and that of caffeine is C9H10N4O2.
(a)
State and explain which method, gas-liquid chromatography (GLC) or high
performance liquid chromatography (HPLC), would be best for the separation
and mass determination of aspirin and caffeine in the tablet.
(2)
- HPLC is the better technique
- component would decompose (because of the higher temperature) in GLC
(b)
State and explain which of the two components would have the shorter
retention time.
(2)
- aspirin because it has a lower molecular mass (~180 vs 206 for caffeine) and
it will move through the HPLC column faster
(c)
Sketch a chromatograph for the separation of the aspirin and caffeine in the
analgesic tablet.
aspirin  lower molecular mass (180 vs 206 for caffeine) so shorter retention time
AND present in greater amount (800 mg vs 40 mg for caffeine) so a higher peak
and greater area under the peak
caffeine  higher molecular mass (206 vs 180 for aspirin( so longer retention time
AND pesent in lower amounts (40 mg vs 800 mg for aspirin) so a lower pear and
smaller area under the peak